Method of transmitting reference signal in wireless communication system

ABSTRACT

A method of transmitting a reference signal in a wireless communication system includes generating a frequency-domain reference signal by performing discrete Fourier transform (DFT) on a time-domain reference signal, generating a transmit signal by performing inverse fast Fourier transform (IFFT) on the frequency-domain reference signal and transmitting the transmit signal.

TECHNICAL FIELD

The present invention relates to wireless communication, and moreparticularly, to a method for transmitting a reference signal in awireless communication system.

BACKGROUND ART

In next generation multimedia mobile communication systems, which havebeen actively studied in recent years, there is a demand for a systemcapable of processing and transmitting a variety of information (e.g.,video and packet data) in addition to the early-stage voice service. Inorder to maximize efficiency of a limited radio resource in a wirelesscommunication system, methods for more effectively transmitting data inspatial and frequency domains have been provided.

Orthogonal frequency division multiplexing (OFDM) uses a plurality oforthogonal subcarriers. Further, the OFDM uses an orthogonality betweeninverse fast Fourier transform (IFFT) and fast Fourier transform (FFT).A transmitter transmits data by performing IFFT. A receiver restoresoriginal data by performing FFT on a received signal. The transmitteruses IFFT to combine the plurality of subcarriers, and the receiver usesFFT to split the plurality of subcarriers. According to the OFDM,complexity of the receiver can be reduced in frequency selective fadingenvironment of a broadband channel, and spectral efficiency can beincreased when selective scheduling is performed in frequency domain byusing channel characteristic which is different from one subcarrier toanother. Orthogonal frequency division multiple access (OFDMA) is anOFDM-based multiple access scheme. According to the OFDMA, efficiency ofradio resources can be increased by allocating different subcarriers tomulti-users.

To maximize efficiency in spatial domain, the OFDM/OFDMA-based systemuses multi-antenna technique which is used as a suitable technique forhigh-speed multimedia data transmission by using a plurality of time andfrequency resources in the spatial domain. The OFDM/OFDMA-based systemalso uses channel coding scheme for effective use of resources in timedomain, scheduling scheme which uses channel selective characteristicamong a plurality of users, hybrid automatic repeat request (HARM)scheme suitable for packet data transmission, etc.

Channel estimation needs to be reliable to ensure high-speed datatransmission. It is important to design a reference signal, which isused for channel estimation, in order to increase the reliability ofchannel estimation. The reference signal is the signal known to both thetransmitter and the receiver, and is also referred to as a pilot. Achannel condition may vary depending on time and frequency. Therefore,the reference signal needs to be designed to cope with the channelcondition flexibly, thereby increasing the reliability of channelestimation.

In general, the reference signal uses a fixed spreading code intime-frequency domain. Orthogonality of the spreading code is used todistinguish users. Since the reference signal is transmitted by usingfixed radio resources, orthogonality between reference signals may beimpaired when the channel condition changes rapidly. This may causeintra-cell interference or inter-cell interference. In this case,channel estimation may be inaccurate. If the same reference signalstructure is used in a situation where channel characteristics of usersare different from one another, it is difficult to increase thereliability of channel estimation. In addition, the reference signal hasto be allocated in a flexible manner in order to increase a systemcapacity because the number of orthogonal codes is limited when a radioresource is limited and also because user accommodation capability isdetermined according to the number of available orthogonal codes.

Therefore, there is a need for a method for designing a reference signalin a flexible manner, whereby the user accommodation capability can beincreased when radio resources are limited and whereby channelestimation and inter-cell interference can be effectively dealt with.

DISCLOSURE OF INVENTION Technical Problem

A method is sought for transmitting a reference signal in a flexiblemanner by using time-frequency resources.

A method is also sought for allocating resources in various manners to areference signal which is used in channel estimation for coherentdetection. The present invention provides a reference signal structurein which the number of available users can be arbitrarily controlled byusing the reference signal. The present invention also provides areference signal structure which is more effective for inter-cellinterference in a multi-cell environment to improve reliability ofchannel estimation.

Technical Solution

In an aspect, a method for transmitting a reference signal in a wirelesscommunication system includes generating a frequency-domain referencesignal by performing discrete Fourier transform (DFT) on a time-domainreference signal, generating a transmit signal by performing inversefast Fourier transform (IFFT) on the frequency-domain reference signaland transmitting the transmit signal.

In another aspect, a method of transmitting data in a wirelesscommunication system includes generating a frequency-domain referencesignal and a frequency-domain data signal by performing discrete Fouriertransform (DFT) on a reference signal and a data signal, generating atransmit signal by performing inverse fast Fourier transform (IFFT) onthe frequency-domain reference signal and the frequency-domain datasignal and transmitting the transmit signal.

Advantageous Effects

Reference signal structure can be designed for various environments andrequirements by using downlink and/or uplink radio resources, and thusefficiency of the radio resources can increase. Further, a PAPR, whichmay be problematic in a conventional OFDM-based system, can be properlydealt with, and thus transmission can be achieved in the same manner assingle carrier transmission. This can directly apply to an SC-FDMAstructure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a structure of a wireless communication system.

FIG. 2 shows an example of a sub-frame.

FIG. 3 shows an example of reference signal allocation in a controlchannel.

FIG. 4 shows an example of radio resource allocation for transmittingreference signals.

FIG. 5 shows another example of radio resource allocation fortransmitting reference signals.

FIG. 6 shows another example of radio resource allocation fortransmitting reference signals.

FIG. 7 shows another example of radio resource allocation fortransmitting reference signals.

FIG. 8 shows another example of radio resource allocation fortransmitting reference signals.

FIG. 9 shows a method for transmitting a reference signal in a frequencydomain according to an embodiment of the present invention.

FIG. 10 is a block diagram showing a transmitter according to anembodiment of the present invention.

FIG. 11 shows transmission of a reference signal by using thetransmitter of FIG. 10.

FIG. 12 is a block diagram of a transmitter according to anotherembodiment of the present invention.

FIG. 13 shows an example of transmission of a control signal by usingthe transmitter of FIG. 12.

FIG. 14 is a block diagram of a transmitter according to anotherembodiment of the present invention.

FIG. 15 shows transmission of a reference signal by using thetransmitter of FIG. 14.

FIG. 16 is a block diagram of a transmitter according to anotherembodiment of the present invention.

MODE FOR THE INVENTION

FIG. 1 shows a structure of a wireless communication system. Thewireless communication system can be widely deployed to provide avariety of communication services, such as voices, packet data, etc.

Referring to FIG. 1, a wireless communication system includes a userequipment (UE) 10 and a base station (BS) 20. The UE 10 may be fixed ormobile, and may be referred to as another terminology, such as a mobilestation (MS), a user terminal (UT), a subscriber station (SS), awireless device, etc. The BS 20 is generally a fixed station thatcommunicates with the UE 10 and may be referred to as anotherterminology, such as a node-B, a base transceiver system (BTS), anaccess point, etc. There are one or more cells within the coverage ofthe BS 20.

Hereinafter, downlink is defined as communication from the BS 20 to theUE 10, and uplink is defined as communication from the UE 10 to the BS20. In the downlink direction, a transmitter may be a part of the BS 20,and a receiver may be a part of the UE 10. In the uplink direction, thetransmitter may be a part of the UE 10, and the receiver may be a partof the BS 20.

FIG. 2 shows an example of a sub-frame. The sub-frame includes aplurality of orthogonal frequency division multiplexing (OFDM) symbolsin time domain and a plurality of subcarriers in frequency domain. Aradio frame is composed of a plurality of sub-frames. For example, oneradio frame may be composed of 10 sub-frames.

Referring to FIG. 2, a sub-frame is divided into two parts, i.e., acontrol region and a data region. The control region is used to carry acontrol signal on a control channel, and the data region is used tocarry user data on a data channel. The control signal is used toindicate radio allocation of the data channel, channel condition andstatus of the data channel. Examples of the control signal include anacknowledgement (ACK)/not-acknowledgement (NACK) signal for hybridautomatic repeat request (HARQ), a channel quality indicator (CQI) toindicate channel condition, a precoding matrix index (PMI) to indicate aprecoding matrix, a rank indicator (RI) to indicate a rank, etc. Thecontrol channel carries only the control signal. The data channel maycarry both the user data and the control signal. Herein, overallbandwidth is divided into three parts so that two parts at both ends areassigned to the control region, and a middle part thereof is assigned tothe data region. The control channel and the data channel have undergonefrequency division multiplexing (FDM). However, this is only forexemplary purposes, and thus there is no limit in arrangement of thecontrol channel and the data channel.

FIG. 3 shows an example of reference signal allocation in a controlchannel.

Referring to FIG. 3, reference signals are allocated over 3 OFDM symbolsand 12 subcarriers. For a radio resource area of a pre-allocatedreference signal, each UE is distinguished by using a spreading code intime and frequency domain. Neighboring cells are allocated withdifferent spreading codes, thereby dispersing influence of inter-cellinterference.

For example, it is assumed that a spreading code for a reference signalis used to achieve orthogonality by performing cyclic shift of aconstant amplitude zero auto-correlation (CAZAC) sequence in frequencydomain, and a discrete Fourier transform (DFT)-based spreading code isused in time domain. Orthogonality can be achieved between cells bychanging a root index of the CAZAC sequence.

When the reference signal is transmitted by using limited radioresources, there is no way but to use a fixed spreading code in fixedtime-frequency domain. The present inventive concept discloses variousreference signal (RS) structures which effectively use time-frequencyresources allocated to transmit the reference signal. A method fortransmitting a reference signal by dividing allocated resources isdisclosed by taking various examples.

FIG. 4 shows an example of radio resource allocation for transmittingreference signals. Herein, radio resources are allocated in a controlchannel.

Referring to FIG. 4, two-dimensional radio resources composed of 5 OFDMsymbols and 12 subcarriers are allocated to the reference signals.Hereinafter, Sf denotes a spreading factor (SF) in frequency domain, andSt denotes an SF in time domain. In addition, Kf denotes the number oforthogonal spreading codes in the frequency domain, and Kt denotes thenumber of orthogonal spreading codes in the time domain. IndependentKf×Kt types of reference signals can be transmitted by using Sf×Sttwo-dimensional radio resources. One two-dimensional radio resource usedin transmission of the reference signals is referred to as a referencesignal (RS) radio resource.

In an embodiment, the RS radio resource can be allocated to each UE soas to be used to distinguish UEs. When one RS radio resource isallocated to one UE, one cell can accommodate a maximum of Kf×Kt UEs. Inanother embodiment, reliability of channel estimation can be increasedby allocating a plurality of the RS radio resources to the UEs. In stillanother embodiment, at least one RS radio resource which is independentin time/frequency domain can be allocated to a specific UE so as to makethe reference signal robust under degraded orthogonality or be allocatedto a specific cell so as to mitigate inter-cell interference. In stillanother embodiment, the plurality of the RS radio resources may bedivided into a plurality of groups. In this case, RS radio resourcesbelonging to a specific group may be allocated to a group of UEs havingsimilar channel condition so that an RS structure suitable for the groupcan be used. Further, UEs may be arbitrarily allocated to each RS radioresource group so as to equalize influence of channel condition on aspecific UE. Allocation of the RS radio resource group between cells canbe used to mitigate inter-cell interference.

It is very important to ensure orthogonality of a spreading code whenchannel estimation is performed by using the reference signals. In acase where the reference signals are distinguished by performingspreading in the time domain and/or the frequency domain, if time orfrequency selective fading channel characteristic changes rapidly in aspecific UE, orthogonality degradation of the specific UE may give aneffect on channel estimation capability of another UE.

FIG. 5 shows another example of radio resource allocation fortransmitting reference signals.

Referring to FIG. 5, radio resources are split in frequency domain,wherein the RS radio resource of FIG. 4 is regarded as a basic unit. Thebasic RS radio resource is split in the frequency domain to form a newRS radio resource. Accordingly, flexibility of code allocation can beensured for each UE.

FIG. 6 shows another example of radio resource allocation fortransmitting reference signals.

Referring to FIG. 6, radio resources are split in time domain, whereinthe RS radio resource of FIG. 4 is regarded as a basic unit. The basicRS radio resource is split in the time domain to form a new RS radioresource. Accordingly, flexibility of code allocation can be ensured foreach UE.

FIG. 7 shows another example of radio resource allocation fortransmitting reference signals.

Referring to FIG. 7, radio resources are split in time-frequency domain,wherein the RS radio resource of FIG. 4 is regarded as a basic unit. Thebasic RS radio resource is split in the time-frequency domain to form anew RS radio resource. Accordingly, flexibility of code allocation canbe ensured for each UE.

FIG. 8 shows another example of radio resource allocation fortransmitting reference signals.

Referring to FIG. 8, radio resources are split in frequency domain,wherein the RS radio resource of FIG. 4 is regarded as a basic unit, andthereafter hopping is performed. The basic RS radio resource is split inthe frequency domain and is then hopped so as to ensure additionalflexibility and to equalize influence of inter-cell interference. Inaddition to a hopping pattern, a cell-specific scrambling code may alsobe used to obtain the same effect.

Although splitting and hopping in the frequency domain have beendescribed herein, hopping and scrambling may apply for splitting in thetime domain and splitting in the time-frequency domain.

FIG. 9 shows a method for transmitting a reference signal in frequencydomain according to an embodiment of the present invention. This showsthat the reference signal to be transmitted is code division multiplexed(CDM) in the frequency domain.

Referring to FIG. 9, an RS radio resource is defined as a column of aDFT matrix, and a reference signal is mapped by using a spreading code.The DFT matrix is an orthogonal matrix, and such a term does not limitthe scope thereof. Each column of the DFT matrix can be used todistinguish UEs. A reference signal for one UE can be transmitted byusing a plurality of columns.

When the reference signal is mapped, mapping is not limited to a radioresource area dedicatedly allocated for the reference signal, and thusthe reference signal may also be used in data transmission. An exampleof a DFT matrix configured such that the reference signal hasorthogonality for each UE in a code area is shown in an upper part ofFIG. 9. An example of a DFT matrix configured such that the referencesignal is transmitted along with data is shown in a lower portion ofFIG. 9.

The aforementioned RS structure can be used in a control signaltransmission method or a data signal transmission method using time orfrequency domain spreading. In the case of frequency-domain CDM, a DFTcolumn vector can be used as a spreading code. Assume that time-domainspreading is used for the purpose of: (a) distinguishing UEs in the samecell; (b) equalizing or reducing inter-cell interference; and (c)distinguishing a data signal and a reference signal, in a case where thedata signal (i.e., control signal or user data) is transmitted usingtime-domain CDM. Then, spreading methods based on the DFT matrix can bemutually complemented as follows. For the purpose of (a), the UEs can bedistinguished within the same symbol by allocating column vectors ofdifferent DFT matrices to the UEs and by using the allocated columnvectors. For the purpose of (b), signals have to be distinguishablebetween cells in a CDM manner within the same resource. For this, thecolumn vectors of the DFT matrix are allocated not to overlap betweenconsecutive cells, thereby being able to reduce inter-cell interference.For the purpose of (c), when the data signal and the reference signalare simultaneously transmitted by one UE, each column vector of the DFTmatrix is allocated, thereby being able to reduce influence ofinter-signal interference.

According to the purposes of spreading, the spreading methods based onthe DFT matrix can be combined so that a control channel can beeffectively configured. By doing so, efficiency of a radio resource canbe increased. For example, a spreading code in time domain may be usedto distinguish UEs and to equalize inter-cell interference, and when thedata signal and the reference signal are intended to be simultaneouslytransmitted in a frequency domain, each UE can configure a controlchannel of a data channel by using a DFT matrix in accordance with thenumber of allocated subcarriers.

In a method for spreading a reference signal in frequency domain,pseudo-noise (PN), an orthogonal code, or a specific sequence (e.g.,CAZAC sequence) having an excellent correlation characteristic isdirectly transmitted by performing spreading in the frequency domain,and in this manner, independent signal processing can be performed.However, when the spreading of the control signal is limited to a timedomain or when priority (e.g., distinguishing UE, equalizing inter-cellinterference, etc.) is given to the spreading, or further, when UEs orcells are distinguished by using frequency division multiplexing (FDM),different spreading codes have to be used in a time-frequency domain dueto a different RS structure and a different control signal transmissionmethod. Therefore, a problem arises in that transmission efficiency ofthe control signal is limited. For example, when a spreading code isdirectly mapped in the frequency domain by using the same RS structure,an SF for control signal spreading in the time domain decreases as moreradio resources are allocated for the reference signal, which has adirect effect on an accommodation capability of UE. Further, inter-cellinterference is also affected. Accordingly, when the control signal istransmitted in the time domain in a CMD manner, there is a need for amethod in which the control signal to be transmitted is effectivelycombined, instead of a method in which independent frequency-domainspreading is used to transmit a reference signal.

According to the present inventive concept, a spreading code used for acontrol signal is used when a reference signal is transmitted. Further,the spreading code is not affected as the number of reference signalsare increased when the control signal is transmitted by using atime-domain CDM method. Furthermore, transmission efficiency of thecontrol signal and throughput of the reference signal can be arbitrarilycontrolled.

FIG. 10 is a block diagram showing a transmitter according to anembodiment of the present invention.

Referring to FIG. 10, a transmitter includes a DFT unit 210 thatperforms DFT and an IFFT unit 220 that performs IFFT. The DFT unit 210performs DFT on an input time-domain reference signal dRS and an inputdata signal dData, and outputs a frequency-domain reference signal and afrequency-domain data signal. The data signal dData may be a controlsignal and/or user data. The IFFT unit 220 performs IFFT on the receivedfrequency-domain reference signal and the received frequency-domain datasignal, and thus outputs Tx signals. The Tx signals are time-domainsignals.

The DFT unit 210 may receive only the time-domain reference signal, ormay receive both the time-domain reference signal and the data signal inparallel and/or series manners. Although it has been described that onetime-domain reference signal is input between two data signals,arrangement between the data signal and the time-domain reference signalis not limited thereto. The time-domain reference signal and the datasignal may be input in a locally concentrated manner or may be input ina spreading manner with a specific interval.

FIG. 11 shows transmission of a reference signal by using thetransmitter of FIG. 10.

Referring to FIG. 11, in order to transmit a data signal and a referencesignal, column vectors of a DFT matrix are allocated to codes fortransmitting the data signal and the reference signal. When the signalsare transmitted in the form of OFDM symbols by using a DFT matrix havinga specific size (i.e., the number of subcarriers to be used), the samestructure as a general single carrier-frequency division multiplexing(SC-FDM) structure is used, which is advantageous to reduce apeak-to-average power ratio (PAPR).

This can be easily applied to a system based on the SC-FDMA (singlecarrier-frequency division multiple access) structure. In particular, itis possible to solve a problem in which a reference signal isindependently allocated in a frequency domain and which occurs when adata signal is transmitted by using time-domain spreading. Thus, thereis an advantage in that efficiency of a limited radio resource can bemore improved than when a frequency-domain data signal is transmitted.

For example, time-domain spreading may be used to distinguish UEs and toreduce inter-cell interference. In addition, when a data signal and areference signal are transmitted base on DFT, data transmission may beperformed by pre-determining the number of reference signals suitablefor coherent transmission.

FIG. 12 is a block diagram of a transmitter according to anotherembodiment of the present invention.

Referring to FIG. 12, a transmitter includes a DFT unit 310, an IFFTunit 320, and a spreading unit 330. The DFT unit 310 performs DFT on aninput time-domain reference signal d_(RS) and an input data signald_(Data), and outputs a frequency-domain reference signal and afrequency-domain data signal. The data signal d_(Data) may be a controlsignal and/or user data. The IFFT unit 320 performs IFFT on the receivedfrequency-domain reference signal and the received frequency-domain datasignal, and thus outputs Tx signals. The Tx signals are time-domainsignals. The spreading unit 330 spreads the Tx signals by using aspreading code so that the Tx signals can be distinguished between UEsor between cells.

FIG. 13 shows an example of transmission of a control signal by usingthe transmitter of FIG. 12.

Referring to FIG. 13, three subcarriers are grouped to form one controlchannel unit. The control channel unit includes one reference signal andtwo data signals. Time-domain spreading uses a CAZAC sequence having alength of seven and using seven symbols. When two control channel unitsare allocated to a specific UE, two reference signals d_(RS,1) andd_(RS,2) and four data signals d_(Data,1), d_(Data,2), d_(Data,3),d_(Data,4) can be transmitted by using a 6×6 DFT matrix. Althoughdifferent control channel units are allocated, each UE can bedistinguished by performing cyclic shift of the CAZAC sequence used in atime domain. Influence of inter-cell interference can be reduced betweenconsecutive cells by using indices of different CAZAC sequences.

In the aforementioned RS structure, the number of reference signalsincreases in proportion to the number of control channel units used byan actual UE. Thus, it can be expected that reliability of channelestimation increases. This can be applied irrespective of a size ofradio resource allocated for data transmission. In addition, a pluralityof UEs can be supported by using frequency domain division, time domaindivision, time-frequency domain division, etc. When data is transmittedby using two control channel units, seven CAZAC sequences can becyclic-shifted by using four reference signals and block-levelspreading, thereby generating a symbol capacity amounting to a total of56 symbols. This results in increase in transmission capacity by 56% incomparison with symbol capacity amounting to 36 symbols in frequencydomain.

Downlink and/or uplink radio resources can be designed for variousenvironments and requirements, and thus efficiency of the radioresources can be increased. Channel estimation can be effectivelyperformed by using a proposed RS structure. A resource can be flexiblyallocated for a reference signal when a control channel or a datachannel is designed. The reference signal can be selectively usedaccording to channel environment and mobility. As the number ofspreading codes increases, accommodation capability of UE increases, andinter-cell interference is mitigated. In particular, when the controlsignal is transmitted under the requirement that reliability isimportant, it is possible to effectively use flexible arrangement of thereference signal.

A PAPR may be problematic in a conventional OFDM-based system, but thisproblem can be solved in the present invention, and thus transmissioncan be achieved in the same manner as single carrier transmission. Thiscan be directly used in the SC-FDMA structure.

FIG. 14 is a block diagram of a transmitter according to anotherembodiment of the present invention.

Referring to FIG. 14, a transmitter includes a spreading unit 410, a DFTunit 420, and an IFFT unit 430. The spreading unit 410 spreads an inputtime-domain reference signal dRS and an input data signal dData by usingan arbitrary spreading code so as to distinguish UEs and/or cells. TheDFT unit 420 performs DFT on the received signals and outputsfrequency-domain signals. The IFFT unit 430 performs IFFT on thereceived frequency-domain signals and outputs Tx signals. The Tx signalsare time-domain signals.

FIG. 15 shows transmission of a reference signal by using thetransmitter of FIG. 14.

Referring to FIG. 15, a control channel includes 6 subcarriers and 7OFDM symbols. The transmitter includes a first spreading unit 510, asecond spreading unit 520, a DFT unit 530, and an IFFT unit 540. Thefirst spreading unit 510 spreads a time-domain reference signal d_(RS,1)and time-domain data signals d_(Data,1) and d_(Data,2) for a first UE byusing an arbitrary spreading code (e.g., a first Walsh code), andoutputs a first spreading signal. The second spreading unit 520 spreadsa time-domain reference signal d_(RS,2) and data signals d_(Data,3) andd_(Data,4) for a second UE by using an arbitrary spreading code (e.g., asecond Walsh code), and outputs a second spreading code. The DFT unit530 performs DFT on the received first and second spreading signals andoutputs frequency-domain signals. The IFFT unit 540 performs IFFT on thereceived frequency-domain signals and outputs Tx signals.

FIG. 16 is a block diagram of a transmitter according to anotherembodiment of the present invention.

Referring to FIG. 16, the transmitter includes a DFT unit 610, aspreading unit 620, and an IFFT unit 630. The DFT unit 610 performs DFTon an input time-domain reference signal d_(RS) and an input data signald_(Data), and outputs frequency-domain signals. The spreading unit 620spreads the frequency-domain signals by using an arbitrary spreadingcode so that UEs and/or cells can be distinguished. The IFFT unit 630performs IFFT on the spread frequency-domain signals and outputs Txsignals.

Reference signal structure can be designed for various environments andrequirements by using downlink and/or uplink radio resources, and thusefficiency of the radio resources can increase. Further, a PAPR, whichmay be problematic in a conventional OFDM-based system, can be properlydealt with, and thus transmission can be achieved in the same manner assingle carrier transmission. This can directly apply to an SC-FDMAstructure.

Every function as described above can be performed by a processor suchas a micro-processor based on software coded to perform such function, aprogram code, etc., a controller, a micro-controller, an ASIC(Application Specific Integrated Circuit), or the like. Planning,developing and implementing such codes may be obvious for the skilledperson in the art based on the description of the present invention.

Although the embodiments of the present invention have been disclosedfor illustrative purposes, those skilled in the art will appreciate thatvarious modifications, additions and substitutions are possible, withoutdeparting from the scope of the invention. Accordingly, the embodimentsof the present invention are not limited to the above-describedembodiments but are defined by the claims which follow, along with theirfull scope of equivalents.

The invention claimed is:
 1. A method of transmitting a reference signalby a user equipment (UE) in a wireless communication system, the methodcomprising: generating, by the UE, an uplink reference signal in asubframe, the uplink reference signal used by a base station todemodulate a hybrid automatic repeat request (HARQ) ACK/NACK signal ofthe UE, the subframe comprising a plurality of orthogonal frequencydivision multiplexing (OFDM) symbols in time domain and a plurality ofsubcarriers in frequency domain; and transmitting, by the UE to the basestation, the uplink reference signal only in a first set of the OFDMsymbols of the subframe; and transmitting, by the UE to the basestation, the HARQ ACK/NACK signal only in a second set of the OFDMsymbols of the subframe different from the first set of the OFDM symbolsof the subframe, wherein the step of generating the uplink referencesignal includes: determining a sequence group from a plurality offrequency sequence groups based on a cell specific hopping parameter;determining a basic frequency spreading sequence based on the determinedsequence group; generating a frequency spreading sequence by cyclicallyshifting the determined basic frequency spreading sequence; andgenerating the uplink reference signal by spreading the frequencyspreading sequence only over the first set of the OFDM symbols with anorthogonal spreading code.
 2. The method of claim 1, wherein the numberof the plurality of subcarriers of the subframe is 12 and the number ofOFDM symbols in the first set of OFDM symbols is
 3. 3. The method ofclaim 2, wherein the OFDM symbols in the first set of OFDM symbols areconsecutive.
 4. A device for transmitting a reference signal in awireless communication system, the device comprising: a transmitterhaving a spreading unit configured to: generate an uplink referencesignal in a subframe, the uplink reference signal used by a base stationto demodulate a hybrid automatic repeat request (HARQ) ACK/NACK signal,the subframe comprising a plurality of orthogonal frequency divisionmultiplexing (OFDM) symbols in time domain and a plurality ofsubcarriers in frequency domain; and transmit the uplink referencesignal only in a first set of the OFDM symbols of the subframe; andtransmit the HARQ ACK/NACK signal only in a second set of the OFDMsymbols of the subframe different from the first set of the OFDM symbolsof the subframe, wherein the transmitter is configured to generate theuplink reference signal by: determining a sequence group from aplurality of frequency sequence groups based on a cell specific hoppingparameter; determining a basic frequency spreading sequence based on thedetermined sequence group; generating a frequency spreading sequence bycyclically shifting the determined basic frequency spreading sequence;and generating the uplink reference signal by spreading the frequencyspreading sequence only over the first set of the OFDM symbols with anorthogonal spreading code.